OMAHA, Neb. — An explosion Monday morning that brought down part of an animal feed processing plant in Omaha left at least two people dead and 10 others seriously hurt, authorities said.

It was unclear if the death toll would rise as crews continued sifting through the rubble of the International Nutrition plant. Interim Omaha Fire Chief Bernie Kanger said the search was progressing slowly because the structure is unsafe.

Kanger wouldn’t provide an exact figure for the number of dead, but Douglas County Attorney Don Kleine said Monday afternoon that two deaths had been confirmed.

Thirty-eight people were working at the plant when the blast happened. In addition to the two people who died and 10 who were hospitalized, seven people were hurt but refused treatment. Officials have not said how many of the 19 others escaped.

“We haven’t cleared the building yet because of the significant risk to our people,” Kanger said.

He said that he didn’t believe anyone who was still in the building Monday afternoon was alive.

A team of urban search-and-rescue experts arrived from Lincoln to help with the search.

Authorities don’t know what caused the blast, but Kanger noted that there were no hazardous chemicals at the plant. The Occupational Safety and Health Administration will determine the cause.

The explosion knocked out the lights in the building and sent workers scrambling for safety.

Nate Lewis said he was on the first floor when he heard the explosion. The building went dark, so the 21-year-old used light from his cellphone to make his way across the production floor and outside.

“I was a production line worker, although I don’t know if I want to be that anymore,” said Lewis, who’s worked at International Nutrition for about four months.

Worker Jamar White said he heard a loud crack and looked up to see the back wall of the building collapsing.

Afterward, White said, he could see inside the third floor of the building where at least two co-workers were screaming for help.

There appears to be structural damage to the top of the building, which sits in an industrial area visible from Interstate 80, which bisects Nebraska’s largest city. There are no residences nearby and no other buildings were evacuated after the explosion.

Diane Stout said she’d heard from her husband, a manager on the plant maintenance crew, so she knew he was OK. The workers all know each other well, Stout said, so she was hoping to hear good news about friends there.

White’s wife, Sarah White, said she was at home with her four children when her husband called her after the explosion.

“I could hear the panic in his voice,” she said. “But he said he was OK.”